Why the 'Simple Deform' modifier bending my object on the wrong axis?
By default, with the Simple Deform modifier, in Bend mode, the object will bend on its Z axis. So if your object bends in a unexpected way, it must be because you have rotated it in Object mode, and its Z axis is not aligned on the global Z axis anymore. In that case you may want to apply the rotation with a ctrlA.

If you give the Simple Deform an other Axis Origin, like for example an empty, the object will bend according to this second object Z axis. So same thing, if your object bends in a unexpected way, rotate your empty the way you want.